LONDON — Police, politicians and activists in Britain are warning of rising anti-Muslim sentiment after the slaughter of an off-duty British soldier in a London street, an apparent act of Islamic extremism that has horrified the nation.

Metropolitan Police investigating the killing of Lee Rigby, a 25-year-old soldier who was run over by attackers then butchered with knives, arrested three more men in the murder investigation Saturday. Stun guns were used on two of the three men, aged 24 and 28, police said.

The latest arrests came as an estimated 1,500 members of an extremist right-wing group called the English Defense League marched in the northern English city of Newcastle, chanting Rigby’s name.

In the southern English city of Portsmouth, police arrested two men for a racially motivated assault as hundreds of demonstrators gathered near one mosque, while several more people were detained for alleged racist offenses elsewhere.

The two men suspected of killing the soldier, Michael Adebolajo, 28, and Michael Adebowale, 22, remained under armed guard in separate London hospitals after police shot them at the scene.

Police have not officially named the suspects because they have not been charged, but British officials, speaking on condition of anonymity because they are not authorized to discuss the investigation, have confirmed their names.

The murder Wednesday in southeast London’s Woolwich area shocked the nation. The horrific scenes were recorded on witnesses’ cellphones, and a video picked up by British media showed one of the two suspects, his hands bloodied, making political statements and warning of further violence as the soldier lay on the ground behind him.

Counterterrorism police are questioning a friend of Adebolajo who was arrested Friday night after he gave BBC Television an interview detailing why he thought Adebolajo might have become radicalized.

Metropolitan Police said the friend, identified by the BBC as 31-year-old Abu Nusaybah, was wanted himself on suspicion of “the commission, preparation or instigation of acts of terrorism.” Police declined to elaborate.

In his BBC interview, Nusaybah said he knew Adebolajo as a moderate Islam convert. He said he thought Adebolajo’s behavior changed after a trip to Kenya last year, and alleged that Britain’s MI5 domestic spy agency tried to recruit him upon his return six months ago.

Rigby’s killing — and Adebolajo’s apparent link to Islamic extremism — has stirred anti-Muslim backlashes across Britain. Police said they arrested three people on suspicion of posting racist tweets ahead of the English Defense League march, and further detained 24 others before and during the protest on suspicion of public drunkenness, vandalism and distributing racist literature.

About 350 counterdemonstrators who called themselves Newcastle Unites shouted abuse at the marchers. The region’s Northumbria Police said riot police prevented any direct clashes between the groups.

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PARIS — A French soldier was stabbed in the throat in a busy commercial district outside Paris on Saturday, and the government said it was trying to determine whether there were any links to the brutal killing of a British soldier.

French President Francois Hollande said the identity of the attacker, who escaped, was unknown. He cautioned against jumping to conclusions about the assault on the uniformed soldier who was on a group patrol in the La Defense shopping area. The life of the 23-year-old soldier was not in danger, the Interior Ministry said in a statement. The Associated Press

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